Conventional gene therapy involves the use of DNA for insertion of desired genetic information into host cells. The DNA introduced into the cell is usually integrated into the genome of one or more transfected cells, allowing for long-lasting action of the introduced genetic material in the host. While there may be substantial benefits to such sustained action, integration of exogenous DNA into a host genome may also have many deleterious effects. For example, it is possible that the introduced DNA will be inserted into an intact gene, resulting in a mutation which impedes or even totally eliminates the function of the endogenous gene. Thus, gene therapy with DNA may result in the impairment of a vital genetic function in the treated host, such as e.g., elimination or deleteriously reduced production of an essential enzyme or interruption of a gene critical for the regulation of cell growth, resulting in unregulated or cancerous cell proliferation. In addition, with conventional DNA based gene therapy it is necessary for effective expression of the desired gene product to include a strong promoter sequence, which again may lead to undesirable changes in the regulation of normal gene expression in the cell. It is also possible that the DNA based genetic material will result in the induction of undesired anti-DNA antibodies, which in turn, may trigger a possibly fatal immune response.
In contrast to DNA, the use of RNA as a gene therapy agent is substantially safer because (1) RNA does not involve the risk of being stably integrated into the genome of the transfected cell, thus eliminating the concern that the introduced genetic material will disrupt the normal functioning of an essential gene, or cause a mutation that results in deleterious or oncogenic effects; (2) extraneous promoter sequences are not required for effective translation of the encoded protein, again avoiding possible deleterious side effects; (3) in contrast to plasmid DNA (pDNA), messenger RNA (mRNA) is devoid of immunogenic CpG motifs so that anti-RNA antibodies are not generated; and (4) any deleterious effects that do result from mRNA based on gene therapy would be of limited duration due to the relatively short half-life of RNA. Moreover in many applications, the transient nature of mRNA transfer to cells, i.e., wherein the duration of any therapeutic effect is limited by the life span of the mRNA and the protein product in the cells, is more desirable than the potentially longer lasting effect achieved using DNA based gene therapy. In addition, it is not necessary for mRNA to enter the nucleus to perform its function, thus avoiding a major barrier to DNA based gene therapy.
One reason that mRNA based gene therapy has not been used more in the past is that mRNA is far less stable than DNA, especially when it reaches the cytoplasm of a cell and is exposed to degrading enzymes. The presence of a hydroxyl group on the second carbon of the sugar moiety in mRNA causes steric hinderance that prevents the mRNA from forming the more stable double helix structure of DNA and thus makes the mRNA more prone to hydrolytic degradation. As a result, until recently, it was widely believed that mRNA was too labile to withstand transfection protocols.
Advances in RNA stabilizing modifications have sparked more interest in the use of mRNA in place of plasmid DNA in gene therapy. Yet, in spite of increased stability of modified mRNA, delivery of mRNA to cells in vivo in a manner allowing for therapeutic levels of protein production is still a challenge, particularly for mRNA encoding full length proteins. Some success has been achieved using viral vectors to introduce mRNA into a host, however mRNA transfection using viral vectors can result in an adverse immune response. In some circumstances, the viral vector may even integrate into the host genome. In addition, production of clinical grade viral vectors is also expensive and time consuming. Targeting delivery of the introduced genetic material using viral vectors can also be difficult to control.
Non-viral delivery of mRNA can be achieved using injection of naked nucleic acids, polyplexes, lipoplexes or liposome entrapped mRNA, biolistic delivery via gene gun, particulate carrier mediated delivery, and electroporation. Non-viral transfection or delivery vehicles are generally less-toxic, less immunogenic, and easier and less expensive to prepare than viral vectors for delivery of mRNA. Certain delivery vehicles, such as cationic lipid or polymer delivery vehicles may also help protect the transfected mRNA from endogenous RNases.
Liposomal delivery of nucleic acids has been employed as a means of effectuating the site-specific delivery of encapsulated plasmid DNA, antisense oligonucleotides, short interfering RNA and microRNA-based therapies. However the efficient, therapeutically effective, delivery of mRNAs to targeted cells and tissues, as well as the subsequent transfection of such targeted cells and tissues remains a technical challenge, particularly for delivery of mRNAs encoding full length proteins. It is important to design liposomal delivery systems that provide sufficient stability to reach desired target cells and the ability to efficiently release their encapsulated materials to such target cells to allow for translation of functional protein at therapeutically effective levels.
Many cationic lipids that are employed to construct such liposomal-based delivery vehicles are toxic to the targeted cells when used to deliver therapeutically effective amounts of the encapsulated agent. Accordingly, the toxicity associated with cationic lipid represents a significant obstacle to their general use as non-viral delivery vehicles, particularly in the quantities necessary to successfully deliver therapeutically effective amounts of mRNA to target cells.
To date, significant progress using mRNA gene therapy has been made in applications, particularly for which low levels of translation has not been a limiting factor, such as immunization with mRNA encoding antigens. Clinical trials involving vaccination against tumor antigens by intradermal injection of naked or protamine-complexed mRNA have demonstrated feasibility, lack of toxicity, and promising results. X. Su et al., Mol. Pharmaceutics 8:774-787 (2011). However, low levels of translation can restrict the exploitation of mRNA based gene therapy in other applications which require higher levels of sustained stability of the mRNA encoded protein to exert a prolonged biological or therapeutic effect.
In addition, because mRNA gene therapy benefits are relatively transient as compared to DNA based gene therapy, repeated administration, and typically by injection, are often required to provide long term effects. Thus, more efficient transfection in vivo and the ability to deliver mRNA noninvasively and/or to mucosal sites would improve the prospects for successful application of mRNA gene therapy.